Multiple data surface optical data storage system

ABSTRACT

An optical data storage system comprises a multiple data surface medium and optical head. The medium comprises a plurality of substrates separated by a light transmissive medium. Data surfaces are located on the substrate surfaces. A layer of a semiconductor material is deposited onto each of the data surfaces. The thickness of the semiconductor layer determines the amount of reflectivity for each of the data surfaces.

This is a continuation of application Ser. No. 08/167,606 filed on Dec.15, 1993 now abandoned which is a in part application of U.S.application Ser. No. 8/079,483 filed Jun. 18, 1993 now U.S. Pat. No.5,381,401 which is a divisional Ser. No. 07/710,226 filed Jun. 04,1991,now U.S. Pat. No. 5,255,562.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to optical data storage systems andmore specifically to a storage system having multiple data storagesurfaces.

2. Description of the Prior Art

Optical data storage systems provide a means for storing greatquantities of data on a disk. The data is accessed by focussing a laserbeam onto the data layer of the disk and then detecting the reflectedlight beam. Various kinds of systems are known. In a ROM (Read OnlyMemory) system, data is permanently embedded as marks in the disk at thetime of manufacture of the disk. The data is detected as a change inreflectivity as the laser beam passes over the data marks. A WORM (WriteOnce Read Many) system allows the user to write data by making marks,such as pits, on a blank optical disk surface. Once the data is recordedonto the disk it cannot be erased. The data in a WORM system is alsodetected as a change in reflectivity.

Erasable optical systems are also known. These systems use the laser toheat the data layer above a critical temperature in order to write anderase the data. Magneto-optical recording systems record data byorienting the magnetic domain of a spot in either an up or a downposition. The data is read by directing a low power laser to the datalayer. The differences in magnetic domain direction cause the plane ofpolarization of the light beam to be rotated one way or the other,clockwise or counterclockwise. This change in orientation ofpolarization is then detected. Phase change recording uses a structuralchange of the data layer itself (amorphous/crystalline are two commontypes of phases) to record the data. The data is detected as changes inreflectivity as a beam passes over the different phases.

Some of these optical disks use thin films to optimize performance. Seefor example IBM TDB, Vol. 33, No. 10B, March 1991, p. 482; Japanesepatent application 61-242356, published Oct. 28, 1986; and Japanesepatent application 4-61045, published Feb. 27, 1992.

In order to increase the storage capacity of an optical disk, multipledata layer systems have been proposed. An optical disk having two ormore data layers may in theory be accessed at different layers bychanging the focal position of the lens. Examples of this approachinclude U.S. Pat. No. 3,946,367 issued Mar. 23, 1976 by Wohlmut, et al.;U.S. Pat. No. 4,219,704 issued Aug. 26, 1980 to Russell; U.S. Pat. No.4,450,553 issued May 22, 1984 to Holster, et al.; U.S. Pat. No.4,905,215 issued Feb. 27, 1990 to Hattori, et al.; U.S. Pat. No.5,097,464 issued Mar. 17, 1992 to Nishiuchi, et al.; U.S. Pat. No.4,829,505 issued May 9, 1989 to Boyd, et al.; U.S. Pat. No. 4,852,077issued Jul. 25, 1989 to Clark, et al.; U.S. Pat. No. 4,845,021 issuedJul. 4, 1989 to Miyazaki, et al.; U.S. Pat. No. 4,682,321 issued Jul.21, 1987 to Takaoka, et al.; U.S. Pat. No. 4,298,975 issued Nov. 3, 1981to Van Der Veen et al.; U.S. Pat. No. 4,737,427 issued Apr. 12, 1988 toMiyazaki, et al.; and Japanese Published Application, 60-202545published Oct. 14, 1985; Japanese Published Application, 63-276732published Nov. 15, 1988 by Watanabe, et al.; and IBM TechnicalDisclosure Bulletin, Vol. 30, No. 2, p. 667, July 1987, by Arter, et al.

The problem with these prior art systems has been that the ability toclearly read the data recorded is very difficult if there is more thanone data layer. Intervening data layers absorb the light and greatlyreduce the signal received from the deeper data layers. An optical datastorage system is needed which overcomes these problems.

SUMMARY OF THE INVENTION

In a preferred embodiment of the invention, an optical data storagesystem comprises an optical disk drive and a multiple data surfaceoptical medium. The medium has a plurality of substrate membersseparated by air spaces. The surfaces of the substrate members which areadjacent to the air spaces are the data surfaces. The data surfaces arecoated with a thin film of a semiconductor material such as amorphoussilicon. The thickness of the film at each data surface is such that theoptical detectors of the disk drive receive the same amount of lightfrom each data surface.

The disk drive comprises a laser for generating a laser beam. An opticaltransmission channel directs the light to the medium. The transmissionchannel includes a focus element for focussing the light onto thedifferent data surfaces and an aberration compensator element to correctfor aberrations due to variations in the effective substrate thickness.A reception channel receives reflected light from the medium. Thereception channel includes a filter element to screen out unwanted lightreflected from data surfaces other than the one to be read. Thereception channel has detectors for receiving the reflected light andcircuitry for generating data and servo signals responsive thereto.

For a fuller understanding of the nature and advantages of the presentinvention reference should be made to the following detailed descriptiontaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an optical data storage system of thepresent invention;

FIG. 2A is a cross-sectional view of an optical medium of the presentinvention;

FIG. 2B is a cross-sectional view of an alternative optical medium;

FIG. 3A is a cross-sectional view of a portion of the optical media ofFIG. 2A;

FIG. 3B is a cross-sectional view of a portion of the optical medium ofFIG. 2B;

FIG. 4A is a graph of index of refraction and extinction coefficientversus wavelength for a typical material;

FIG. 4B is a graph of index of refraction (n) and extinction coefficient(k) of amorphous silicon versus wavelength;

FIG. 5 is a graph of light percentage versus layer thickness for oneembodiment of the present invention;

FIG. 6A is a cross-sectional view of the tracking marks of the medium ofFIG. 2;

FIG. 6B is a cross-sectional view of alternative tracking marks;

FIG. 6C is a cross-sectional view of alternative tracking marks;

FIG. 6D is a cross-sectional view of alternative tracking marks;

FIG. 7 is a schematic diagram of an optical head and medium of thepresent invention;

FIG. 8 is a top view of an optical detector of FIG. 7;

FIG. 9 is a circuit diagram of a channel circuit of the presentinvention;

FIG. 10 is a schematic diagram of a controller circuit of the presentinvention;

FIG. 11A is a graph of tracking error signal versus head displacement;

FIG. 11B is a graph of tracking error signal versus head displacementfor an alternative embodiment;

FIG. 11C is a graph of tracking error signal versus head displacementfor an alternative embodiment;

FIG. 12 is a graph of the focus error signal versus lens displacementfor the present invention;

FIG. 13 is a schematic diagram of a multiple data surface aberrationcompensator of the present invention;

FIG. 14 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 15 is a schematic diagram of an additional alternative embodimentof a multiple data surface aberration compensator of the presentinvention;

FIG. 16 is a top view of the compensator of FIG. 15;

FIG. 17 is a schematic diagram of an additional alternative embodimentof a multiple data surface aberration compensator of the presentinvention;

FIG. 18 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 19 is a cross-sectional view of the lens of FIG. 18;

FIG. 20 is a schematic diagram of an alternative embodiment of anoptical head and medium of the present invention;

FIG. 21 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 22 is a schematic diagram of an alternative embodiment of amultiple data surface aberration compensator of the present invention;

FIG. 23 is a schematic diagram showing the process of manufacturing thecompensator of FIGS. 21 and 22;

FIG. 24 is a schematic diagram of an alternative embodiment of theaberration compensator of the present invention;

FIG. 25 is a schematic diagram of an alternative embodiment of theaberration compensator of the present invention;

FIG. 26 is a schematic diagram of a multiple data surface filter of thepresent invention;

FIG. 27 is a schematic diagram of an alternative embodiment of amultiple data surface filter of the present invention;

FIG. 28 is a schematic diagram of an alternative embodiment of amultiple data surface filter of the present invention; and

FIG. 29 is a schematic diagram showing the process of manufacturing thefilter of FIG. 28.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic diagram of an optical data storage system ofthe present invention and is designated by the general reference number10. System 10 includes an optical data storage medium 12 which ispreferably disk shaped. Medium 12 is removably mounted on a clampingspindle 14 as is known in the art. Spindle 14 is attached to a spindlemotor 16 which in turn is attached to a system chassis 20. Motor 16rotates spindle 14 and medium 12.

An optical head 22 is positioned below medium 12. Head 22 is attached toan arm 24 which in turn is connected to an actuator device, such as avoice coil motor 26. Voice coil motor 26 is attached to chassis 20.Motor 26 moves arm 24 and head 22 in a radial direction below medium 12.

The Optical Medium

FIG. 2A is a cross-sectional view of medium 12. Medium 12 has asubstrate 50. Substrate 50 is also known as the face plate or coverplate and is where the laser beam enters medium 12. An outer diameter(OD) rim 52 and an inner diameter (ID) rim 54 are attached between faceplate 50 and a substrate 56. An OD rim 58 and an ID rim 60 are attachedbetween substrate 56 and a substrate 62. An OD rim 64 and an ID rim 66are attached between substrates 62 and a substrate 68. An OD rim 70 andID rim 72 are attached between substrates 68 and a substrate 74. Faceplate 50 and substrates 56, 62, 68 and 74 are made of a lighttransmissive material such as polycarbonate or other polymer material orglass. In a preferred embodiment, face plate 50 and substrates 56, 62,68 and 74 are 0.3 mm thick. The substrates may alternatively havethicknesses of 0.01 to 0.80 mm. The ID and OD rims are preferably madeof a plastic material and are approximately 200 microns thick. The rimsmay alternatively have thicknesses of 10-500 microns. The face plate,substrates and rims are preferably made of polycarbonate and are formedby a molding process.

The rims may be attached to the face plate and substrates by means ofglue, cement, ultrasonic bonding, solvent bonding, or other bondingprocess. The rims may alternatively be integrally formed in thesubstrates during the molding process. When in place, the rims form aplurality of annular spaces 78 between the substrates and the faceplate. A spindle aperture 80 passes through medium 12 inside the ID rimsfor receiving the spindle 14. A plurality of passages 82 are provided inthe ID rims connecting the aperture and the spaces 78 to allow pressureequalization between the spaces 78 and the surrounding environment ofthe disk file, which would typically be air. A plurality of lowimpedance filters 84 are attached to passages 82 to preventcontamination of spaces 78 by particulate matter in the air. Filters 84may be quartz or glass fiber. Passages 82 and filters 84 couldalternatively be located on the OD rim. Alternatively, the rims may beattached with a porous cement which allows air the pass through butfilters out the contaminants. Another alternative, is to ultrasonicallyspot bond the rims leaving small gaps between the bonded areas which arelarge enough to pass air but small enough to filter out particles.

Surfaces 90, 92, 94, 96, 98, 100, 102 and 104 are data surfaces and lieadjacent spaces 78. These data surfaces contain ROM data (of the CD,OD-ROM or CD-ROM format for example) which is formed directly into thesubstrate surfaces as pits or other marks.

Although FIG. 2A illustrates a medium of the present invention which haseight data surfaces, it should be understood that the medium maycomprise any number of a plurality of data surfaces. Additionalsubstrates and rims may be added or subtracted. For example, medium 12may comprise only two data surfaces 90 and 92 by using only face plate50, rims 52 and 54, and substrate 56. In this embodiment, face plate 50and substrate 56 may both be the same thickness, preferably 1.2 mm.

FIG. 2B is a cross-sectional view of an alternative embodiment of ahighly transmissive optical recording medium and is designated by thegeneral reference number 120. Elements of medium 120 which are similarto elements of medium 12 are designated by a prime number. Medium 120does not have the rims and spaces 78 of medium 12. Instead, a pluralityof solid transparent members 122 separates the substrates. In apreferred embodiment, the members 122 are made of a highly transmissiveoptical cement which also serves to hold the substrate together. Thethickness of members 122 is preferably approximately 10-500 microns.Medium 120 may be substituted for medium 12 in system 10. Medium 120 mayalso be made of different members of data surfaces by adding orsubtracting substrates and transparent members. For example, a two datasurface medium comprises face plate 50', member 122 and substrate 56'.Face plate 50' and substrate 56' may both have the same thickness,preferably 1.2 mm.

FIG. 3A shows a detailed cross-sectional view of a portion of disk 12 ofFIG. 2A. Substrate 50 contains the embedded information in the datasurface 90 and is covered by a thin film layer 124. Layer 124 is made ofa material which exhibits low light absorption at or near the wavelengthof a light used in the optical system. For light in the range of 400-850nm in wavelength, materials such as semiconductors are used for layer124. The thickness of thin film layer 124 is in the range 25-5000 Å.Layer 124 is preferably sputtered onto surface 90.

Layer 124 may be covered by an optional protective layer 126. Layer 126may be made of a dielectric such as silicon nitride or polycarbonate.Layer 124 and 126 are sputtered onto substrate 90 after the informationpits and tracking groves have been formed into the substrate.Alternatively, the layers 126 may be spin coated.

Substrate 56 with data surface 92 also has a layer 124 and a protectivelayer 126. The other data surfaces 94, 96, 98, 100, 102 and 104, havesimilar coatings of thin film layers 124 and protective layers 126. Thedata surface at the greatest depth (i.e. the data surface furthest awayfrom the optical head) may substitute a very high reflectivity layer forthe thin film layer 124. This reflective layer may be made of a metalsuch as aluminum, gold or an aluminum alloy which is deposited bysputtering or evaporation.

The protective layers 126 prevent dust, contamination, and moisture,which may be present in the airspace 78 from adversely affecting thelayer 124 and the data surfaces. The protective layers 126 are optionaland may be omitted depending upon the operating requirements. Thethickness of the protective layers in the range of 50 Å to 100 microns.

FIG. 3B shows a detailed cross-sectional view of a portion of the disk120 of FIG. 2B The layers 124' are deposited onto data surfaces 90' and92', respectively. The member 122 separates the layers 124'. There is noneed for a protective layer in this embodiment because member 122 servesas the protective layer.

The thin film layers 124 are used to provide desired amounts of lightreflectivity at each data surface. However, because there are multipledata surfaces through which the light passes, the thin film layers 124must also be highly transmissive and absorb as little light as possible.These conditions can be met when the index of refraction (n) is greaterthan the extinction coefficient (k) and particularly when the index ofrefraction (n) is relatively high (n>1.5) and the extinction coefficient(k) is relatively low (k<0.5). Such conditions occur in certainmaterials over certain frequency ranges. One region where theseconditions can be met is on the high wavelength side of an anomalousdispersion absorption band.

FIG. 4A shows a graph of index of refraction (n) and extinctioncoefficient (k) versus wavelength for a typical material which has ananomalous dispersion absorption band. Anomalous dispersion is generallydefined to be the region where dn/dλ is positive. A more detaileddiscussion of anomalous dispersion is given in M. Born and E. Wolf,"Principles of Optics", Pergamon Press, 3rd Edition, 1964. In thepresent invention, the region of interest is where n>k. As can be seenfrom FIG. 4A, this region occurs at wavelengths above those whereinanomalous dispersion region occurs. Materials which exhibit thisphenomenon strongly include semiconductors.

Semiconductors are materials such as silicon and germanium which areconductive in the presence of certain heat, light or voltage parameters.Amorphous silicon has been found to be a good material for use as layer124 where light in the wavelength range of 400-850 nm is used.

FIG. 4B shows a graph of index refraction (n) and extinction coefficient(k) versus wavelength for amorphous silicon. Note the large range ofwavelength where n is much larger than k. In these light ranges,amorphous silicon will transmit and reflect light without absorbing muchof the light. Amorphous silicon is a good material to use as layer 124.

Other semiconductor materials in addition to amorphous silicon may beused for layer 124. Any of the group IVA elements from the periodictable may be used such as C, Si, Ge, Sn, Pb, or combinations thereof. Inaddition, combinations of group IIIA and VA elements producesemiconductor materials which may be used. These include alloys such asA_(x) B_(1-x), where A is at least one element selected from the groupIIIA elements consisting of B, Al, Ga, In, and Tl; B is at least oneelement selected from the group VA elements consisting of N, P, As, Sband Bi; and 0<x<1. These include GaAs, AlAs, AlP, AlSb, GaP, GaN, GaSb,InP, InAs, and InSb and combinations thereof.

Ternary and quaternary compounds comprised of elements from these IIIA,IVA, and VA groups are also applicable.

These semiconductor materials are deposited as layer 124 in a sputteringprocess. The semiconductor materials are in their natural amorphousstate and this is preferable. Alternatively, after being deposited intheir natural amorphous state, the semiconductor materials may bechanged to a crystalline state by an annealing process.

FIG. 5 shows a graph of percentages of 780 nm wavelength lightreflected, transmitted, and absorbed versus amorphous silicon thickness.Semiconductor materials have relatively good reflectivity and lowabsorption compared to other materials. Low absorption is important in amultiple data surface medium where a fraction of the light will be lostat each intervening layer.

FIG. 5 shows that the amorphous silicon has reflectivity,transmissivity, and absorption percentages which have large sinusoidalvariations over a range of thicknesses. By selecting the properthickness, it is possible to obtain a large number of differentcombinations of reflectivity, transmissivity, and absorption. Thisallows the media of the present invention to be tuned, by varying thethickness of the layer 124, such that the optical head of the disk drivewill receive the same amount of light from each data surface. In otherwords, the thickness of the layer 124 of the deeper data surfaces ischosen to have a higher reflectivity than the reflectivity of the datasurfaces lying closer to the outer surface of the medium. This higherreflectivity is needed to compensate for the losses encountered by thelight in the intervening layers. The end result is that from theperspective of the optical head, the same amount of light will appear tobe reflected from each layer.

Let t_(n), a_(n), and r_(n) denote the transmissivity, absorptivity andreflectivity, respectively of the nth layer. Thus, they can have valuesof between zero and one. Let N denote the total number of layers. Thenthe following recursion relationship holds if the effective reflectivityfrom each layer is identical, (in other words, the same amount of lightis detected at the optical head independent of layer), ##EQU1## wheren+1≦N. The absorption of each layer is kept low to minimize loss oflight. Then the following approximation holds:

    r.sub.n =1-t.sub.n -a.sub.n ≈1-t.sub.n.            (2)

There is a maximum reflectivity r_(max) which can be achieved from theinnermost layer (layer N) which is dictated by the optical constants andthickness of the coating on that layer. For semiconductors, this istypically less than 70%. For metals, this can be as high as 98%. Thereflectivity of the innermost layer plus the desired total number oflayers determines the maximum effective reflectivity which can beachieved identically for each layer.

Using the above equations, the following relation must be true for alllayers. ##EQU2##

This is exact in the limit of no absorption and approximate when a smallamount of absorption is present. For example, if the Nth layer has areflectivity of 35% then t_(N-1) =78% This equation can be used todetermine the thicknesses of all of the layers which will give equalreflectivity as are illustrated in the following tables. First onechooses the reflectivity r_(max) of the Nth (innermost) layer. Then, thetransmission and reflection of the N-1 layer is calculated usingequation (3) to give similar effective reflectivity as the Nth layer.This process is repeated until the properties of the n=1 layer areobtained. The actual reflectivity of layer 1 is the same as theeffective reflectivities of all the inner layers.

The following Tables each show an example of a medium of the presentinvention wherein the effective reflectivity from each data surface isequal. In all of the tables, layer 1 refers to the layer at the datasurface which is closest to the optical head.

                  TABLE 1                                                         ______________________________________                                        Embodiment using Medium 12 having two data surfaces.                          Each of which is covered by a layer 124 made of amorphous silicon.                  Effective          Trans- Ab-                                                 reflectivity                                                                           Reflectivity                                                                            mission                                                                              sorption                                            of       of        of     of     Thickness of                           Data  individual                                                                             individual                                                                              individual                                                                           individual                                                                           reflecting                             Surface                                                                             layer    layer     layer  layer  layer                                  #     [%]      [%]       [%]    [%]    [Angstrom]                             ______________________________________                                        1     26       26        64     10     125                                    2     26       65        29     6      400                                    ______________________________________                                    

                  TABLE 2                                                         ______________________________________                                        Embodiment using a medium 12 having four data surfaces.                       The data surfaces one through three are covered by a layer 124                made of amorphous silicon and the last data surface is                        covered by an aluminum layer 124.                                                   Effective          Trans- Ab-                                                 reflectivity                                                                           Reflectivity                                                                            mission                                                                              sorption                                            of       of        of     of     Thickness of                           Data  individual                                                                             individual                                                                              individual                                                                           individual                                                                           reflecting                             Surface                                                                             layer    layer     layer  layer  layer                                  #     [%]      [%]       [%]    [%]    [Angstrom]                             ______________________________________                                        1     12       12        80.9    7.1.  66                                     2     12       18        76.8   5.2    80                                     3     12       30.3      59.1   10.6   142                                    4     12       90        0      10     300                                    ______________________________________                                    

                  TABLE 3                                                         ______________________________________                                        Embodiment using a medium 12 having six data surfaces..                       The data surfaces one through seven are covered by a layer 124                made of crystalline Si.sub.0.25 Ge.sub.0.75 and the last data surface is      covered by an aluminum layer 124.                                                   Effective          Trans- Ab-                                                 reflectivity                                                                           Reflectivity                                                                            mission                                                                              sorption                                            of       of        of     of     Thickness of                                 individual                                                                             individual                                                                              individual                                                                           individual                                                                           reflecting                             Layer layer    layer     layer  layer  layer                                  #     [%]      [%]       [%]    [%]    [Angstrom]                             ______________________________________                                        1     11.9     11.9      87.9   0.2    50                                     2     11.9     13.3      85.6   0.1    60                                     3     11.9     16.9      82.9   0.2    70                                     4     11.9     23.4      76.5   0.1    90                                     5     11.9     34.9      64.8   0.3    126                                    6     11.9     83        --     --     300                                    ______________________________________                                    

                  TABLE 4                                                         ______________________________________                                        Embodiment using a medium 12 having eight data surfaces..                     The data surfaces one through seven are covered by a layer 124                made of crystalline Al.sub.0.42 Ga.sub.0.58 As.sub.1.0 and the last data      surface is covered by an aluminum layer 124.                                        Effective          Trans- Ab-                                                 reflectivity                                                                           Reflectivity                                                                            mission                                                                              sorption                                            of       of        of     of     Thickness of                                 individual                                                                             individual                                                                              individual                                                                           individual                                                                           reflecting                             Layer layer    layer     layer  layer  layer                                  #     [%]      [%]       [%]    [%]    [Angstrom]                             ______________________________________                                        1     7.15     7.15      92.83  0.02   53                                     2     7.15     8.28      91.71  0.01   63                                     3     7.15     9.85      90.12  0.02   80                                     4     7.15     12.14     87.85  0.01   96                                     5     7.15     15.67     84.30  0.03   120                                    6     7.15     22.01     77.96  0.03   160                                    7     7.15     36.05     63.91  0.04   250                                    8     7.15     88        --     --     300                                    ______________________________________                                    

Although it is preferable to adjust the thickness of layers 124 toachieve the same effective reflectivity for each layer 124, the layers124 may alternatively be made of the same thickness. This may bedesirable in order to simplify the manufacturing process. In this case,the effective reflectivity from each layer 124 will be different.However, the optical drive may adjust the laser power and/or theamplification of the optical detectors in order to compensate for thedifferent effective reflectivities of each layer 124.

In the case of layers 124 having the same thickness, the transmission(t) and reflectivity (r) are the same for all layers 124. The effectivereflectivity for the nth layer is

    r.sub.n (eff)=t.sup.2(n-1) r

Therefore if a lower limit is set on r_(n) (eff) and the values for tand r known as determined by the layer thickness, then the maximumnumber of layers can be determined. ##EQU3##

For example, if layer 124 has r=12%, t=87.5% and the minimum effectivereflectivity is 4%, then the maximum number of layers (data surfaces) isn=5.

The following Table 5 shows an example of a medium of the presentinvention which has four data surfaces. The data surfaces one throughthree are covered by a layer 124 of amorphous silicon of the samethickness, and data surface four is covered by an aluminum layer 124.

                  TABLE 5                                                         ______________________________________                                        (Four Data Surfaces, Different Effective Reflectivity)                               Effective                                                                              Reflect- Trans- Ab-                                                  reflectivity                                                                           ivity    mission                                                                              sorption                                             of       of       of     of     Thickness of                                  individual                                                                             individual                                                                             individual                                                                           individual                                                                           reflecting                             Layer  layer    layer    layer  layer  layer                                  #      [%]      [%]      [%]    [%]    [Angstrom]                             ______________________________________                                        1      17.5     17.5     73.8   8.7    90                                     Amor-                                                                         phous                                                                         Silicon                                                                       2      11.2     17.5     73.8   8.7    90                                     Amor-                                                                         phous                                                                         Silicon                                                                       3      5.2      17.5     73.8   8.7    90                                     Amor-                                                                         phous                                                                         Silicon                                                                       4      13.8     85       0      15     300                                    Aluminum                                                                      ______________________________________                                    

FIG. 6A shows an exaggerated detailed cross-sectional view of a datasurface pattern of medium 12 and is designated by the general referencenumber 130. Surface 90 contains a pattern of spiral (or alternativelyconcentric) tracking grooves 132. The portions of surface 90 locatedbetween the grooves 132 are known as the land portions 134. Surface 92contains a pattern of spiral inverse tracking grooves (raised ridges)136. The portion of surface 92 located between the inverse grooves 136is the land 138. The grooves 132 and the inverse grooves 136 are alsoreferred to as tracking marks. In a preferred embodiment, the widths 140of the tracking marks are 0.6 microns and the width 142 of the landsections is 1.0 microns. This results in a pitch of (1.0+0.6)=1.6microns.

The tracking marks are used to keep the light beam on track while themedium 12 rotates. This is described in more detail below. For pattern130, a beam 144 from the optical head 22 will track on the land portion134 or 138 depending upon which surface it is focussed upon. Therecorded data is on the land portions. In order for the tracking errorssignal (TES) to be of equal magnitude for both surfaces 90 and 92 theoptical path difference between light reflected from the lands andtracking marks must be the same for both surfaces. Beam 144 focuses onsurface 90 through substrate 50, however, beam 144 focuses on surface 92through space 78. In the preferred embodiment space 78 contains air. Forthe optical path length difference between the lands and tracking marksto be the same d1n1 must equal d2n2 (or d2/d1 equals n1/n2), where d1 isthe depth of mark 132 (perpendicular distance), n1 is the index ofrefraction of substrate 50, d2 is the height of mark 136 (perpendiculardistance), and n2 is the index of refraction of space 78. In a preferredembodiment, space 78 contains air which has an index of refraction of1.0 and substrate 50 (as well as the other substrates) has an index ofrefraction 1.5. So the ratio of d2/d1 equals 1.5. In a preferredembodiment, d1 is 700 Angstroms and d2 is 1050 Angstroms. The samepattern of tracking marks is repeated on the other surfaces of medium12. The other substrate incident surfaces 94, 98 and 102 are similar tosurface 90 and the other space incident surfaces 96, 100 and 104 aresimilar to surface 92.

The above discussion of groove depth ratio is applicable to theembodiments of medium 12 which do not have any protective layer 126 orwhich have a thin protective layer 126 which is less than or equal toapproximately 200 Angstroms thick. If the protective layer 126 isgreater or equal to approximately 1 micron thick, then the formulad1n1=d2n2 is still used, but now n2 is the index of refraction of layer126 on the second data surface. If the thickness of the protective layer126 is between approximately 200 Angstroms and 1 micron thick, theninterference phenomenon make the calculation of groove depth moredifficult, However, the proper groove depth can be determined using thethin film optical calculations as shown in M. Born and E. Wolf,"Principles of Optics", Pergamon Press, 3rd Edition, 1964.

Although the tracking marks are preferably arranged in a spiral pattern,they may alternatively be in a concentric pattern. In addition, thespiral pattern may be the same for each data surface, i.e., they are allclockwise or counter-clockwise spirals, or they may alternate betweenclockwise and counter-clockwise spiral patterns on consecutive datalayers. This alternating spiral pattern may be preferable for certainapplications, such as storage of video data, movies for example, wherecontinuous tracking of data is desired. In such a case, the beam tracksthe clockwise spiral pattern inward on the first data surface until thespiral pattern ends near the inner diameter, and then the beam isrefocused on the second data surface directly below and then the beamtracks the counter-clockwise spiral pattern outward until the outerdiameter is reached.

FIG. 6B shows an exaggerated detailed cross-sectional view of analternative surface pattern for medium 12 and is designated by thegeneral reference number 150. Pattern 150 is similar to pattern 130except that the tracking marks for surface 92 are grooves 152 instead ofinverse grooves. The pitch and the ratio of d2/d1 are the same as forpattern 130. Beam 144 will track on land 134 on surface 90, but now beam144 will track on groove 152 when focussed on surface 92. Tracking inthe groove 132 may be desirable in certain situations. However, as willbe described below, beam 144 may also be electronically controlled totrack on land 138 of surface 92. The tracking marks for surfaces 94, 98and 102 are similar to surface 90 and the surfaces 96, 100 and 104 aresimilar to surface 92.

FIG. 6C shows an exaggerated detailed cross-sectional view of analternative surface pattern for medium 12 which is designated by thegeneral reference number 160. Pattern 160 is similar to pattern 130except that surface 90 has inverse grooves 162 instead of grooves 132,and surface 92 has grooves 164 instead of inverse grooves 136. The pitchand ratio of d2/d1 are the same as for pattern 130. Beam 144 will trackon inverse grooves 162 when focussed on surface 90 and will track ongrooves 164 when focussed on surface 92 (unless it is electronicallyswitched to track on the land). The pattern for surfaces 94, 98 and 102are similar to surface 90 and the surfaces 96, 100 and 104 are similarto surface 92.

FIG. 6D shows an exaggerated detailed cross-sectional view of analternative surface pattern designated by the general reference number170. In pattern 170, the surface 90 has a similar structure to surface90 of pattern 160. Surface 92 has a similar structure to surface 92 ofpattern 130. The pitch and ratio of d2/d1 is the same as for pattern130. Beam 144 will track on inverse grooves 162 when focussed on surface90 (unless it is electronically switched to track on the land) and willtrack on land 138 when focussed on surface 92. Surfaces 94, 98 and 102have similar patterns to surface 90 and surfaces 96, 100 and 104 havepatterns similar to surface 92.

For all of the patterns 130, 150, 160 and 170 the tracking marks areformed into the substrate at the time of manufacture by injectionmolding or photopolymer processes as are known in the art. It should benoted that the thin film layers, as described above, are deposited ontothe substrates after the tracking marks are formed.

The discussion of tracking marks is also applicable to other features ofoptical disks. For example, some ROM disks such as CD-ROM, use pitsembossed in the substrate to record data and/or provide trackinginformation. Other optical media use pits to emboss sector headerinformation. Some media use these header pits to also provide trackinginformation. In using such media in the multiple data surface form ofthe present invention, the pits are formed as pits or inverse pits onthe various data surfaces corresponding in a similar manner to thetracking marks discussed above. The optical path length between thelands and the pits or inverse pits is also similar to the trackingmarks. The pits, inverse pits, grooves and inverse grooves are alllocated at a different elevation from the land (i.e. the perpendiculardistance between these items and the land), and are all referred to asmarks for purposes of this discussion. Marks which are specificallydedicated to providing tracking information are known as nondatatracking marks.

It should be understood that the medium of the present invention may bemade in any type of optical disk format such as CD, CD-ROM or OD-ROM.These formats are well known in the art.

The Optical Head

FIG. 7 shows a schematic diagram of an optical head 22 and medium 12.Optical head 22 has a laser diode 200. Laser 200 may be agallium-aluminum-arsenide diode laser which produces a primary beam oflight 202 at approximately 780 nanometers wavelength. Beam 202 iscollimated by lens 203 and is circularized by a circularizer 204 whichmay be a circularizing prism. Beam 202 passes to a beamsplitter 205. Aportion of beam 202 is reflected by beamsplitter 205 to a focus lens 206and an optical detector 207. Detector 207 is used to monitor the powerof beam 202. The rest of beam 202 passes to and is reflected by a mirror208. Beam 202 then passes through a focus lens 210 and a multiple datasurface aberration compensator 212 and is focused onto one of the datasurfaces (surface 96 as shown) of medium 12. Lens 210 is mounted in aholder 214. The position of holder 214 is adjusted relative to medium 12by a focus actuator motor 216 which may be a voice coil motor.

A portion of the light beam 202 is reflected at the data surface as areflected beam 220. Beam 220 returns through compensator 212 and lens210 and is reflected by mirror 208. At beamsplitter 205, beam 220 isreflected to a multiple data surface filter 222. The beam 220 passesthrough filter 222 and passes to a beamsplitter 224. At beamsplitter 224a first portion 230 of beam 220 is directed to an astigmatic lens 232and a quad optical detector 234. At beamsplitter 224 a second portion236 of beam 220 is directed through a half-wave plate 238 to apolarizing beamsplitter 240. Beamsplitter 240 separates light beam 236into a first orthogonal polarized light component 242 and a secondorthogonal polarized light component 244. A lens 246 focuses light 242to an optical detector 248 and a lens 250 focuses light 244 to anoptical detector 252.

FIG. 8 shows a top view of a quad detector 234. The detector 234 isdivided into four equal sections 234A, B, C and D.

FIG. 9 shows a circuit diagram of a channel circuit 260. Circuit 260comprises a data circuit 262, a focus error circuit 264 and a trackingerror circuit 266. Data circuit 262 has an amplifier 270 connected todetector 248 and an amplifier 272 connected to detector 252. Amplifiers270 and 272 are connected to a double pole, double throw electronicswitch 274. Switch 274 is connected to a summing amplifier 276 and adifferential amplifier 278.

Circuit 264 has a plurality of amplifiers 280, 282, 284 and 286connected to detector sections 234A, B, C and D, respectively. A summingamplifier 288 is connected to amplifiers 280 and 284, and a summingamplifier 290 is connected to amplifiers 282 and 286. A differentialamplifier 292 is connected to summing amplifiers 288 and 290.

Circuit 266 has a pair of summing amplifiers 294 and 296, and adifferential amplifier 298. Summing amplifier 294 is connected toamplifiers 280 and 282, and summing amplifier 296 is connected toamplifiers 284 and 286. Differential amplifier 298 is connected tosumming amplifiers 294 and 296 via a double pole double throw electronicswitch 297. Switch 297 acts to invert the inputs to amplifier 298.

FIG. 10 is a schematic diagram of a controller system of the presentinvention and is designated by the general reference number 300. A focuserror signal (FES) peak detector 310 is connected to the focus errorsignal circuit 264. A track error signal (TES) peak detector 312 isconnected to the tracking error signal circuit 266. A controller 314 isconnected to detector 310, detector 312, detector 207 and circuits 262,264 and 266. Controller 314 is a microprocessor based disk drivecontroller. Controller 314 is also connected to and controls the laser200, head motor 26, spindle motor 16, focus motor 216, switches 274 and297, and compensator 212. The exact configuration and operation ofcompensator 212 is described in more detail below.

The operation of system 10 may now be understood. Controller 314 causesmotor 16 to rotate disk 12 and causes motor 26 to move head 22 to theproper position below disk 12. See FIG. 7. Laser 200 is energized toread data from disk 12. The beam 202 is focussed by lens 210 on the datasurface 96. The reflected beam 220 returns and is divided into beams230, 242 and 244. Beam 230 is detected by detector 234 and is used toprovide focus and tracking servo information, and beams 242 and 244 aredetected by detectors 248 and 252, respectively, and are used to providedata signals.

See FIG. 8. When beam 202 is exactly focussed on data surface 96, beam230 will have a circular cross-section 350 on detector 234. This willcause circuit 264 to output a zero focus error signal. If beam 202 isslightly out of focus one way or the other, beam 230 will fall as anoval pattern 352 or 354 on detector 234. This will cause circuit 264 tooutput a positive or negative focus error signal. Controller 314 willuse the focus error signal to control motor 216 to move lens 210 untilthe zero focus error signal is achieved.

If beam 202 is focussed exactly on a track of data surface 96, then beam230 will fall as a circular cross-section 350 equally between thesections A and B, and the sections D and C. If the beam is off track itwill fall on the boundary between a tracking mark and the land. Theresult is that the beam is diffracted and cross-section 350 will move upor down. More light will be received by sections A and B, and less bysections C and D or vice versa.

FIG. 11A shows a graph of the TES produced by circuit 264 versus thedisplacement of head 22. Controller 314 causes VCM 26 to move head 22across the surface of medium 12. TES peak detector 312 counts the peaks(maximum and minimum points) of the TES signals. There are two peaksbetween each track. By counting the number of peaks, controller 314 isable to position the beam on the proper track. The TES signal at a landis a positive slope TES signal. Controller 314 uses this positive slopesignal to lock the beam on track. For example, a positive TES signalcauses head 22 to move to the left toward the zero point land positionand a negative TES signal causes the head 22 to move to the right towardthe zero point land position. FIG. 11A shows the signal derived from thepreferred pattern 130 of medium 12 when switch 297 is in its initialposition as shown in FIG. 9. The same signal is also generated forsurface 90 of pattern 150, and surface 92 of pattern 170. The beam isautomatically locked to the land because that is the position wherethere is a positive slope.

FIG. 11B shows a graph of the TES versus head displacement for surface92 of pattern 150, surfaces 90 and 92 of pattern 160 and surface 90 ofpattern 170 when switch 297 is in its initial position. Note that inthis case the tracking marks are such that the positive slope signaloccurs at the location of the tracking marks and so that the beam willautomatically track on the tracking marks and not the land portions.Tracking on the tracking marks may be desirable in some circumstances.

FIG. 11C shows a graph of the TES versus head displacement for surface92 of pattern 150, surfaces 90 and 92 of pattern 160 and surface 90 ofpattern 170 when inverter switch 297 is enabled such that the TES signalis inverted. The TES now has a positive slope at the land positions andthe beam will track on the land portion instead of the tracking marks.Thus, controller 314 can track the grooves or the lands by settingswitch 297.

Medium 12 contains ROM data surfaces. Reflectivity detection is used toread the ROM data. In data circuit 262, switch 274 is positioned toconnect amplifier 276 when a ROM disk is to be read. The signal fromdetectors 248 and 252 is added. Less light is detected where data spotshave been recorded and this difference in light detected is the datasignal. Switch 274 will have the same setting for reading WORM and phasechange data disk. If a disk is used which has magneto-optical datasurfaces, then polarization detection is needed to read the data. Switch274 will be set to connect amplifier 278. The difference in theorthogonal polarization light detected at detectors 248 and 252 willthen provide the data signal.

FIG. 12 shows a graph of the focus error signal from circuit 264 versusthe displacement distance of lens 210. Note that a nominally sinusoidalfocus error signal is obtained for each of the data surfaces of medium12. Between the data layers, the focus error signal is zero. Duringstartup of the system, controller 314 first causes motor 216 to positionlens 210 at its zero displacement position controller 314 will then seekthe desired data surface by causing motor 216 to move lens 210 in apositive displacement direction. At each data layer, peak detector 310will detect the two peaks of the focus error signal. Controller 314 willcount the peaks (two per data surface) and determine the exact datasurface on which beam 202 is focussed. When the desired surfaces arereached, controller 314 causes motor 216 to position lens 210 such thatthe focus error signal is between the two peaks for that particular datasurface. The focus error is then used to control the motor 216 to seekthe zero point focus error signal between the peaks, i.e. lock on thepositive slope signal such that exact focus is achieved. The controller314 will also adjust the power of laser 200, the switch 297, and theaberration compensator 212 as appropriate for that particular datasurface.

Also on startup, controller 314 determines what type of disk it isreading. Switch 274 is first positioned for reflectivity detection andswitch 297 is set to read the land portions of the disk of the preferredpattern 130. The controller 314 seeks and reads the header informationof the first track of the first data surface. The header has informationon the number of layers, what type of optical media is in each layer(reflectivity or polarization detection), and what type of tracking markpatterns are used. With this information, the controller 314 is able toset switches 274 and 297 to correctly read each data surface.

If controller 314 is unable to read the first track of the first datasurface (perhaps the first layer has a different tracking mark pattern),then controller 314 will set switch 297 to its other setting and willattempt to read the first track of the first data surface again. If thisstill does not work (perhaps the first data surface is magneto-optic andrequires polarization detection) then the controller will set switch 274to the polarization detection and try again, setting switch 297 at onesetting and then the other. In summary, controller 314 will read theheader information of the first track of the first data surface bytrying the four different combinations of settings of switches 274 and297 until it is successful at reading the track. Once controller 314 hasthis header information, it can correctly set the switches 274 and 297for each of the other data surfaces.

Alternatively, the disk drive may be specifically dedicated to work withonly the ROM medium of the present invention. In that case, controller314 is preprogrammed to store information on the type of data surfaces,number of layers, and types of tracking marks.

The Aberration Compensator

Lenses are typically designed to focus light through air which has anindex of refraction of 1.0. When such lenses focus light throughmaterials having different indices of refraction, the light experiencesa spherical aberration, which distorts and enlarges the beam spot,degrading the reading and recording performance.

In typical optical data storage systems, there is only one data surfaceonto which to focus. The data surface is usually located beneath a 1.2mm thick face plate. The lens is typically a 0.55 numerical aperture(NA) lens which is specially designed to correct for sphericalaberration caused on the light by the 1.2 mm face plate. The result isthat a good spot focus can be obtained at that exact depth, but at otherdepths the focus gets blurry. This causes severe problems for anymultiple data layer system.

The aberration compensator 212 of the present invention solves thisproblem. FIG. 13 shows a schematic diagram of an aberration compensatorwhich is designated by the general reference number 400 and may be usedas compensator 212. Compensator 400 comprises a stepped block 402 havingthree steps. A first step 404 has a thickness of 0.3 mm, a second step406 has a thickness of 0.6 mm and a third step 408 has a thickness of0.9 mm. The block 402 is made of the same material as the face plate andsubstrates of medium 12 or other similar optical material. Note thatthese steps increase in optical thickness in increments of the substratethickness. Block 402 is attached to a voice coil motor 410 (or similaractuator device) which in turn is connected to controller 314. Motor 410moves block 402 laterally into and out of the path of beam 302.

Lens 210 is designed to focus on the lowest data surface of medium 12.In other words, lens 210 is designed to compensate for sphericalaberrations caused by the combined thicknesses of the face plate and theintervening substrates. For the present invention, in order to focus onsurface 102 or 104, beam 202 must pass through the face plate 50 andsubstrates 56, 62 and 68 (a combined thickness of 1.2 mm of thesubstrate material). Note that the air spaces 78 are not counted becausethey impart no additional spherical aberration. Lens 210 is thusdesigned to focus through 1.2 mm of polycarbonate and may focus equallywell on both data surfaces 102 and 104.

When beam 202 is focussed on either surface 102 or 104, the block 402 iscompletely withdrawn and beam 202 does not pass through it. When beam202 is focussed on surface 98 or 100, block 402 is positioned such thatbeam 202 passes through step 404. When beam 202 is focussed on surfaces94 or 96, block 402 is positioned such that beam 202 passes through step406. When beam 202 is focussed on surfaces 90 or 92, block 402 ispositioned such that beam 202 passes through step 408. The result isthat no matter which pair of surfaces are focussed on, beam 202 willalways pass through the same total optical thickness of material andwill not experience spherical aberration problems. Controller 314controls motor. 410 to move the block 402 as appropriate.

FIG. 14 shows an aberration compensator which is designated by thegeneral reference number 430 and which may be used for compensator 212.Compensator 430 has a pair of complementary triangular shaped blocks 432and 434. Blocks 432 and 434 are made of the same material as face plateand substrates of medium 12 or material of similar optical properties.Block 432 is positioned in a fixed position such that beam 202 passesthrough it. Block 434 is attached to a voice coil motor 436 and may beslid along the surface of block 432. Controller 314 is connected to andcontrols motor 436. By moving block 434 relative to block 432 theoverall thickness of material through which beam 202 passes may beadjusted. The result is that beam 202 passes through the same opticalthickness of material no matter which data surface it is focussed on.

FIGS. 15 and 16 show an aberration compensator which is designated bythe general reference number 450 and may be used for compensator 212.Compensator 450 has a circular stepped element 452. Element 452 has foursections 454, 456, 458 and 460. Sections 456, 458 and 460 havethicknesses similar to steps 404, 406 and 408, respectively, ofcompensator 400. Section 454 has no material and represents a blankspace in the circular pattern as shown in FIG. 16. The circular element452 is attached to a stepper motor 462 which in turn is controlled bycontroller 314. Spindle 462 rotates element 452 such that beam 202passes through the same thickness of material no matter which datasurface it is focussed on.

FIG. 17 shows an aberration compensator which is designated by thegeneral reference number 570 and may be used for compensator 212.Compensator 570 comprises a stationary convex lens 572 and a moveableconcave lens 574. Lens 574 is attached 16 a voice coil motor 576. Voicecoil motor 576 is controlled by controller 314 to move lens 574 relativeto lens 572. Beam 202 passes through lens 572, lens 574 and lens 210 tomedium 12. Moving lens 574 relative to lens 572 changes the sphericalaberration of beam 202 and allows it to focus on the different datasurfaces. In a preferred embodiment lenses 210, 574 and 572 comprise aCooke triplet having movable center element 574. Cooke triplets aredescribed in more detail in the article by R. Kingslake, "Lens DesignFundamentals," Academic Press, New York, 1978, pp. 286-295. Althoughlens 574 is shown as the moving element, alternatively, lens 574 couldbe stationary and lens 572 used as the moving element. In FIG. 4 theaberration compensator 212 is shown between lens 210 and medium 12.However, if compensator 570 is used it will be located between lens 210and mirror 208 as shown in FIG. 17.

FIG. 18 shows an aberration compensator which is designated by thegeneral reference number 580. Compensator 580 comprises an aspheric lenselement 582 with nominally zero focal power. Element 582 has a sphericalaberration surface 584 and a planar surface 586. Lens 582 is connectedto a voice coil motor 588. Voice coil motor 588 is controlled bycontroller 314 which moves lens 582 relative to lens 512. Beam 202passes through lens 210 and lens 582 to medium 12. Moving lens 582relative to lens 210 changes the spherical aberration of the beam 202and allows it to focus on the different data surfaces.

FIG. 19 shows a view of lens 582 relative to axes z and ρ. In apreferred embodiment, the surface of 584 should correspond to theformula Z=0.00770ρ⁴ -0.00154ρ⁶.

FIG. 20 shows a schematic diagram of an alternative optical head of thepresent invention and is designated by the general reference number 600.Elements of head 600 which are similar to elements of head 22 aredesignate by a prime number. Note that head 600 is similar to system 10except that the aberration compensator 212 has been eliminated and a newaberration compensator 602 has been added between beamsplitter 206' andmirror 208'. The description and operation of compensator 602 isdescribed below. The operation of head 600 is otherwise the same asdescribed for head 22. Head 600 may be substituted for head 22 in system10.

FIG. 21 shows a schematic diagram of an aberration compensator which isdesignated by the general reference number 610 and may be used forcompensator 602. Compensator 610 comprises a substrate 612 having areflective holographic coating 614. Substrate 612 is attached to astepper motor 616 which in turn is controlled by controller 314.Holographic coating 614 has a number of different holograms recorded,each of which imparts a particular spherical aberration to beam 202'.These holograms are of the Bragg type which are sensitive only to lightincident at a specific angle and wavelength. When substrate 612 isrotated a few degrees, beam 202' will experience a different hologram.The number of holograms recorded corresponds to the number of differentspherical aberration corrections required. For medium 12 as shown, fourdifferent recordings are necessary each corresponding to one of thepairs of data surfaces.

FIG. 22 shows a schematic diagram of an aberration compensator which isdesignated by the general reference number 620 and may be used forcompensator 602. Compensator 620 comprises a substrate 622, atransmissive holographic coating 624 and a stepper motor 626. Thecompensator 620 is similar to compensator 610 except that here theholographic coating 624 is transmissive rather than reflective.Holographic coating 624 has a number of holograms recorded, each ofwhich corresponds to the amount of spherical aberration compensationrequired. Beam 202' experiences each of these holograms in turn assubstrate 622 is rotated.

FIG. 23 shows a schematic diagram of a recording system used to make theholographic coatings 614 and 624, and is designated by the generalreference number 650. System 650 has a laser 652 which produces a lightbeam 654 at a frequency similar to the laser 200. Light 654 iscollimated by lens 656 and is passed to a beamsplitter 658. Beamsplitter658 divides the light into a beam 660 and a beam 662. Beam 660 isreflected by a mirror 664 and 666, and is focussed by a lens 668 to apoint 670 in a plane 672. Beam 660 passes through a stepped block 674similar to block 402. Beam 660 is then recollimated by a lens 676 andfalls upon a holographic coating 680 on a substrate 682. Substrate 682is rotatably mounted to a stepper motor 684. Beam 662 also falls uponcoating 680 at a 90 degree angle from beam 660.

Lens 668 forms an unaberrated spot on plane 672. This light is thenpassed through a step of block 674 which has a thickness representingthe sum of the substrate thicknesses which will be encountered inaccessing a particular recording layer. Lens 676 is identical in designto lens 210 as used in the optical storage head. It collimates the lightinto a beam that contains a specific amount of spherical aberrationcorresponding to the specific thickness. This wavefront isholographically recorded by interference with the reference beam 662. Ifthe hologram is oriented in approximately a plane 690 as shown, atransmission hologram is recorded. If it is oriented in approximately aplane 692 as shown as a dash line, a reflective hologram is recorded.The wavefront required to correct the aberrations encountered inaccessing a different pair of recording layers is holographically storedby rotating the hologram to a new angular position and inserting thecorresponding thickness plate of block 674. A multiplicity or angularlyresolved holograms are recorded, each corresponding to and providingcorrection for a different pair of recording layers. The holographiccoating may be made of dichromatcd gelatin or a photopolymer material.The individual holograms can be recorded in increments as small as onedegree without appreciable cross-talk. This permits large numbers ofholograms to be recorded and correspondingly large numbers of datasurfaces to be used.

FIG. 24 shows a schematic diagram of an alternative aberrationcompensator which is designated by the general reference number 700 andmay be used for compensator 602. Compensator 700 comprises a polarizingbeamsplitter 702, a quarter waveplate 704, a carousel 706 attached to astepper motor 708 and a plurality of spherical aberration mirrors 710each providing a different spherical aberration correction. Beam 202' isoriented with its polarization such that it passes through beamsplitter702 and plate 704 to one of mirrors 710. Mirror 710 imparts theappropriate spherical aberration to the beam 202' which then returnsthrough plate 704 and is reflected by beamsplitter 702 to mirror 208'.Motor 708 is controlled by controller 314 to rotate the carousel 706 toposition the appropriate mirror in place. Mirrors 710 are reflectingSchmidt corrector plates. See M. Born, et at., "Principles of Optics,"Pergamon Press Oxford, 1975, pp. 245-249.

FIG. 25 shows a schematic diagram of an aberration compensator which isdesignated by the general reference number 720 and may be used forcompensator 602. Compensator 720 comprises a polarizing beamsplitter722, a quarter waveplate 724 and an electrical controlled deformablemirror 726. Deformable mirror 726 is controlled by internalpiezo-electric elements and is described in more detail in J. P.Gaffarel, et at., "Applied Optics," Vol. 26, pp. 3772-3777, (1987). Theoperation of compensator 720 is similar to compensator 700, except thatmirror 726 is electrically adjusted to provide the appropriate sphericalaberration. In other words, mirror 726 is adjusted to form a reflectivesurface corresponding to the different Schmidt corrector plates 710 ofcompensator 700. Controller 314 controls the adjustment of mirror 726 asappropriate.

The operation of the aberration compensators 212 and 602 have beendescribed above in connection with medium 12. Due to the air spacebetween the layers, one aberration compensation setting will work foreach pair of data surfaces. However, in the case where medium 120 isused, aberration compensation settings will need to be made for eachdata surface. This is because there are no air spaces.

Multiple Data Surface Filter

When beam 202 is focussed on a particular data surface of medium 12 areflected beam 230 is returned to head 22 from that surface. However,some of light beam 202 is also reflected at the other data surfaces.This unwanted reflected light must be screened out for proper data andservo signals to be obtained. The multiple data surface filter 222 ofthe present invention achieves this function.

FIG. 26 shows a schematic diagram of a filter 750 which may be used asfilter 222. Filter 750 comprises a blocking plate 754 and a lens 756.The desired light beam 230 is collimated because it is the light whichhas been properly focussed by lens 210. Beam 230 is focussed by lens 752to a point 760. Unwanted light 762 is not properly focussed by lens 210and is thus not collimated. The light 762 will not focus to point 760.Plate 754 has an aperture 764 at point 760 which allows light 230 topass. Most of the unwanted light 762 is blocked by plate 754. The light230 is recollimated by lens 756. In a preferred embodiment aperture 764is circularly shaped and has a diameter of approximately λ/(2*(NA)),where λ is the wavelength of the light and (NA) is the numericalaperture of lens 752. The exact diameter is determined by the desiredtrade-off between alignment tolerances and interlayer signal rejectionrequirements. Alternatively, aperture 764 may be a slit having a minimumgap distance of approximately λ/(2*(NA)). In such a case plate 764 couldbe two separate members which are separated by the slit. Plate 754 maybe made of a metal sheet or may be made of a transparent substratehaving a light blocking coating with aperture 764 being uncoated.

FIG. 27 shows a schematic diagram of a filter 800 which also may be usedas filter 222. Filter 800 comprises a lens 802, a blocking plate 804, ablocking plate 806 and a lens 808. Plate 806 has an aperture 810 locatedat a focal point 812 of lens 802. Plate 804 has a complementary aperture814 which allows the collimated light 230 to be directed throughaperture 810 while blocking unwanted uncollimated light 820. Aperture814 may be a pair of parallel slits or an annular aperture. In apreferred embodiment, the distance between the slits of aperture 814 isgreater than the diameter of aperture 810. The diameter of aperture 810is approximately equal to λ/(2*(NA)). For the alternative annular shapedaperture, the inner diameter of the annular slit should be greater thanthe diameter of aperture 810. In both cases, the outer edge 822 ofaperture 814 is located outside of beam 230. Blocking plates 804 and 806may be made of a metal sheet or may be made of a transparent substratehaving a light blocking coating with apertures 810 and 814 beinguncoated.

FIG. 28 shows a schematic diagram of an alternative filter 830 which maybe used as filter 222. Filter 830 comprises a beamsplitter 832 and aholographic plate 834. The coating on the holographic plate 834 is tunedto efficiently reflect collimated beam 230 while uncollimated beam 840is allowed to pass. The desired beam 230 is reflected from holographicplate 834 and returns to beamsplitter 832 where it is reflected towardsbeamsplitter 224.

FIG. 29 is a schematic diagram which shows how holographic plate 834 ismade. A collimated laser beam 850 having approximately the samewavelength as laser 200 is split into two beams 852 and 854 at anamplitude beamsplitter 856. Beams 852 and 854 are directed by mirrors860 and 862, respectively, and fall upon hologram plate 834 fromopposite directions perpendicular to the surface of plate 834. Areflective hologram is recorded by the interference of beams 852 and854. The holographic coating may be made of a dichromated gel orphotopolymer material.

Filters 222 of the present invention have been shown in FIG. 6 to belocated in the path of beam 220. However, one or more filters can belocated in the separate paths of servo beam 230 or the data beam 236.

While the preferred embodiments of the present invention have beenillustrated in detail, it should be apparent that modifications andadaptations to those embodiments may occur to one skilled in the artwithout departing from the scope of the present invention as set forthin the following claims.

We claim:
 1. A multiple data layer optical data storage disk for use inan optical disk drive of the type having a laser for generating a singlewavelength laser light beam, a focusing lens for directing a spot oflaser light from the beam to any one of a plurality of spaced apart datalayers, and an optical reception device for receiving laser lightreflected from any one of the data layers, the disk comprising:a firstmember transmissive to the single wavelength laser light and having afirst data layer for storing recorded data as marks formed in the firstdata layer, the first data layer having a layer of a semiconductormaterial deposited thereon; a second member having a second data layerfor storing recorded data as marks formed in the second data layer; asupport device for supporting the first and second data layers in aspaced apart relationship; a medium transmissive to the singlewavelength laser light and located between the first and second datalayers; wherein the layer of semiconductor material on the first datalayer has a predetermined thickness enabling transmission of a portionof the single wavelength laser light from the laser to said second datalayer and reflection of light from said second data layer to saidoptical reception device when the focused spot is located on said seconddata layer; and said predetermined thickness enabling reflection of aportion of the single wavelength laser light from said first data layerto said optical reflection device when the focused spot is located onsaid first data layer.
 2. The disk of claim 1, wherein the transmissivemedium is air.
 3. The disk of claim 1, wherein the transmissive mediumis a solid transparent material.
 4. The disk of claim 3, wherein thetransmissive medium functions as the support device.
 5. The disk ofclaim 1, wherein the second data layer has a layer of a semiconductormaterial deposited thereon.
 6. The disk of claim 1, wherein the datalayers are ROM data surfaces.
 7. The disk of claim 1, wherein thesemiconductor material is comprised of at least one element from thegroup consisting of C, Si, Ge, Sn, Pb.
 8. The disk of claim 1, whereinthe semiconductor material is amorphous silicon.
 9. The disk of claim 1,wherein the semiconductor material is A_(x) B_(1-x), where A is at leastone element from the group consisting of B, Al, Ga, In, and Tl, B is atleast one element from the group consisting of N, P, As, Sb, and Bi, and0<x<1.
 10. The disk of claim 1, wherein the semiconductor material isGaAs.
 11. The disk of claim 1, wherein the semiconductor material isAlAs.
 12. The disk of claim 1, wherein the semiconductor material isAlP.
 13. The disk of claim 1, wherein the semiconductor material isAlSb.
 14. The disk of claim 1, wherein the semiconductor material isGaP.
 15. The disk of claim 1, wherein the semiconductor material is GaN.16. The disk of claim 1, wherein the semiconductor material is GaSb. 17.The disk of claim 1, wherein the semiconductor material is InP.
 18. Thedisk of claim 1, wherein the semiconductor material is InAs.
 19. Thedisk of claim 1, wherein the semiconductor material is InSb.
 20. Thedisk of claim 1, wherein the semiconductor material has an index ofrefraction (n) and an extinction coefficient (k) such that n>1.5 andk<0.5.
 21. The disk of claim 1, wherein the predetermined thickness ofthe layer of semiconductor material is in the range of 25-5000Angstroms.
 22. The disk of claim 1, further comprising at least oneadditional data surface.
 23. The disk of claim 1, further comprising ametal reflective layer deposited onto the second data surface.
 24. Thedisk of claim 1, wherein the second member is a laser light radiationtransmissive member and has a third data layer opposite the second datalayer and further comprising a third member having a fourth data layerand a second laser light radiation transmissive medium located betweenthe third and fourth data layers.
 25. The disk of claim 24, wherein thesecond and third data layers have a semiconductor layer depositedthereon.
 26. The disk of claim 24, further comprising a metal reflectivelayer deposited onto the fourth data layer.
 27. The disk of claim 1,further comprising a dielectric protective layer overlying thesemiconductor layer.
 28. The disk of claim 1, wherein the semiconductormaterial layer has a thickness such that the total effectivereflectivity of the single wavelength laser light from each of the firstand second data layers is substantially the same.
 29. An optical datastorage system comprising:a laser radiation source for generating laserlight at a generally fixed wavelength; an optical medium comprising afirst laser radiation transmissive member having a first data surfacefor storing recorded data as marks formed in the first data surface, alayer of a semiconductor material deposited on the first data surface, asecond data surface for storing recorded data as marks formed in thesecond data surface, and a solid laser radiation transmissive mediumlocated between the first and second data surfaces for spacing the datasurfaces apart; an optical transmission device for directing laser lightfrom the laser radiation source through said first member to any one ofsaid data surfaces, said device including a lens for locating a focusedspot of laser light to any one of said data surfaces; an opticalreception device for receiving a reflected laser light from any one ofsaid data surfaces and providing a data signal responsive thereto; andwherein the layer of semiconductor material on said first data surfacehas a predetermined thickness enabling transmission of the laser lightat said fixed wavelength from the laser radiation source to said seconddata surface and reflection of light from said second data surface tosaid optical reception device when the focused spot is located on saidsecond data surface, said predetermined semiconductor layer thicknessenabling reflection of laser light at said fixed wavelength from saidfirst data surface to said optical reflection device when the focusedspot is located on said first data surface.
 30. The system of claim 29,wherein the second data surface has a layer of a semiconductor materialdeposited thereon.
 31. The system of claim 29, wherein the data surfacesare ROM data surfaces.
 32. The system of claim 29, wherein thesemiconductor material is comprised of at least one element from thegroup consisting of C, Si, Ge, Sn, Pb.
 33. The system of claim 29,wherein the semiconductor material is amorphous silicon.
 34. The systemof claim 29, wherein the semiconductor material is A_(x) B_(1-x), whereA is at least one element from the group consisting of B, Al, Ga, In,and Tl, B is at least one element from the group consisting of N, P, As,Sb, and Bi, and 0<x<1.
 35. The system of claim 29, wherein thesemiconductor material is GaAs.
 36. The system of claim 29, wherein thesemiconductor material is AlAs.
 37. The system of claim 29, wherein thesemiconductor material is AlP.
 38. The system of claim 29, wherein thesemiconductor material is AlSb.
 39. The system of claim 29, wherein thesemiconductor material is GaP.
 40. The system of claim 29, wherein thesemiconductor material is GaN.
 41. The system of claim 29, wherein thesemiconductor material is GaSb.
 42. The system of claim 29, wherein thesemiconductor material is InP.
 43. The system of claim 29, wherein thesemiconductor material is InAs.
 44. The system of claim 29, wherein thesemiconductor material is InSb.
 45. The system of claim 29, wherein thesemiconductor material has an index of refraction (n) and an extinctioncoefficient (k) such that n>1.5 and k<0.5.
 46. The system of claim 29,wherein the predetermined thickness of the layer of semiconductormaterial is in the range of 25-5000 Angstroms.
 47. The system of claim29, further comprising at least one additional data surface.
 48. Thesystem of claim 29, further comprising a metal reflective layerdeposited onto the second data surface.
 49. The system of claim 29,further comprising a dielectric protective layer overlying thesemiconductor layer.
 50. The system of claim 29, wherein thepredetermined thickness of the semiconductor material layer allows thetotal effective reflectivity of the fixed wavelength laser light fromeach of the first and second data layers is substantially the same.